The development of a portable, easy to use instrument for the direct measurement of the alpha activity weighted particle size distributions in indoor environments composed primarily of radon decay progeny is proposed. Our methods for achieving this are based on the use of wire screen collectors and low cost alpha particle detectors to completely characterize the size distribution of the radio-aerosols over the particle size range 0.3 - 500 nm. Our measurement technique will allow the simultaneous measurement of the alpha particle energy spectrum of the size selected aerosols which are sequentially deposited on wire screen filters in the detector. The activity weighted particle size distribution can then be inferred from these measurements using an unfolding technique. Inhaled radon progeny, particularly those due to the "unattached" fraction, are estimated to be the dominant sources of background radiation that deliver significant doses to the tissues or cells of the respiratory tract. Phase I will result in the construction, testing, and calibration of a laboratory working model of the particle size sampler and the demonstration that a transportable monitor based on wire screen arrays and low cost solid state alpha particle detectors and associated electronics can be reliably utilized. During Phase II we will configure the technology into a relatively inexpensive system which will be readily transportable for real time measurement of activity weighted radon progeny particle size distributions in environments where complete characterization of the alpha activity is required. Phase III will involve commercialization of this technology through fabrication and sales of various stand-alone instruments based on the technology.